ACHROMATIC CONDENSERS 109 



Such a complete series is only made by a few manu- 

 facturers, notably Swift, and Watson, others supply the 

 two or three most generally useful, corresponding to 

 Nos. 1, 3, and 5. 



They are intended for use in the following way : 



Low-Power Condensers. 



No. 1 evenly illuminates objects up to about I" in 

 diameter for planar lenses. 



No. 2 is to be used with low-power objectives up to 

 about y. 



Dry Condensers. 



Nos. 3 and 4 have the same N.A., and most makers 

 only provide one condenser of that aperture, but Swift, 

 and Watson make two. Swift's " Paragon " and Watson's 

 " Universal " represent No. 3, and have larger back lenses 

 than the " Achromatic " and " Parachromatic " of the 

 same makers corresponding to No. 4. The latter are of 

 higher power and corrected with greater perfection, but 

 require greater skill to use them to their best advantage ; 

 they pass less light than the other two. These condensers 

 are intended for use with medium- and high-power 

 objectives, and with their front lens removed with low 

 powers. 



Oil-Immersion Condensers are for use with oil-immersion 

 objectives, when the widest possible cone of light is 



Fig. 36 



APLANATIC OIL-IMMERSION CONDENSER BY LEITZ 



required. A reference to Fig. 21 will show the advantage 



